The present invention relates to a multi-tone recording method for a charge control and multi-step deflection control type ink jet printer which selectively charges an ink drop by a plurality of charging voltages upon separation of the drop from ink issuing from a nozzle and, then, selectively deflects charged drop in a plurality of steps by a deflection electric field based on the amount of charge deposited thereon.
A typical prior art ink jet printer employs thirty-two consecutive steps for deflection so as to selectively charge ink drops in thirty-two different steps and, thereby, prints out data by thirty-two dots in a vertical array, for example. In this case, a carriage with an ink ejection head and others mounted thereon is moved in a horizontal direction so that data may be recorded over a width of thirty-two dots by a single printing stroke of the carriage.
For gradation recording, or halftone recording, the ink jet printer of the type described operates on the basis of a unit dot matrix having four dots in the longitudinal (vertical) direction and four dots in the transverse (horizontal) direction (one pattern consisting of a 4.times.4 dot matrix), representing sixteen different densities by determining whether or not to print out on each of the dot positions. Even such an approach may fail to provide a desired number of tones. Then, assuming a region wherein two dots are to be printed out in the longitudinal direction, the recording density, or density determined by a recording area relative to a predetermined area, is higher when two dots are printed out overlapping each other in the longitudinal direction as shown in FIG. 1B than when a single dot is printed out as shown in FIG. 1A. Further, in the case of the two dots, the resulting recording density is higher when they are printed out without overlapping as shown in FIG. 1C. Such provides the following six different patterns when considered in a matrix pattern having two dots longitudinally and two dots transversely. In matrix patterns shown below, dot positions labeled "0" represent non-print positions and those labeled "1", print positions. When "1" is followed by another "1" in the longitudinal or transverse direction, the dots overlap; however, when it is aligned with another "1" diagonally, the dots do not overlap. The recording density sequentially increases from the leftmost pattern to the rightmost.
______________________________________ 0 0 1 0 1 0 1 0 1 1 1 1 0 0 0 0 1 0 0 1 0 1 1 1 ______________________________________
As shown above, the 2.times.2 dot pattern provides six different tones which is larger than four. It follows that a larger number of tones are attainable by employing such a dot diameter and a dot pitch which allow dots to overlap in the longitudinal and transverse directions but not in the diagonal one, and using such tone patterns with the dot distributions shown in FIGS. 1B and 1C. However, there is a keen demand for extra tones for the representation of halftones.